Referring to "the explosion of a motorcycle", they said in a statement that "four citizens were killed and four others injured while two vehicles near the motorcycle caught fire".

There was no immediate claim of responsibility for the explosion.

The blast hit at a crossroads in central Basra, near a hospital and a popular market, an AFP photographer reported.

He saw a car charred and left largely a shell, its doors gone. Shrapnel pierced a minibus whose windows were smeared with blood.

Iraq has been torn by years of war and insurgency since a US-led invasion toppled dictator Saddam Hussein in 2003.

In recent years, the city of Basra has mostly been spared such attacks.

But since late November deadly attacks claimed by the Islamic State (IS) group have targeted Kurdish fighters in northern Iraq.

Prime Minister Mustafa al-Kadhemi called for a rapid investigation to identify who carried out the "cowardly terrorist attack" in Basra.

He added on Twitter that security forces "will not permit the security of the people to be threatened."

IS established a self-proclaimed "caliphate" across Syria and Iraq from 2014 but lost its territories to offensives in both countries.

Iraq proclaimed its victory against the jihadists on December 9, 2017.

Today, IS maintains a largely clandestine presence in Iraq and Syria and conducts a sustained insurgency on both sides of the border, according to a UN report published early this year.

Across the two countries, IS is believed to retain about 10,000 active fighters, according to the report.

The last major attack claimed by IS in Iraq targeted a market in Baghdad's Shiite neighbourhood of Sadr City in July and killed about 30 people.